1. Field of the Invention
The present invention relates to an optical receiver.
2. Related Prior Art
Various optical receivers for the optical communication have been proposed. For example, the U.S. Pat. No. 6,624,918 has disclosed one type of the optical receiver, in which the optical receiver including the light-receiving device, the preamplifier, and the digital controller has been disclosed. The digital controller in this optical receiver, responding the control signal externally providing thereto, adjusts the bandwidth of the preamplifier through the voltage regulator that supplies the power to the preamplifier. In the practical operation, the controller first presets the bandwidth of the preamplifier. When the bandwidth is narrow, the controller adjusts the voltage regulator to increase the supply voltage to the preamplifier, which shifts the 3 dB cut-off frequency of the preamplifier to the higher frequency side. On the other hand, when the preset bandwidth is unnecessary wide, the controller adjusts the voltage regulator to decrease the supply voltage to the preamplifier.
Japanese Patent Application published as 2003-244075 has disclosed another optical receiver with the bandwidth thereof dynamically optimized. This optical receiver includes a trans-impedance amplifier and a bandwidth adjuster connected to the trans-impedance amplifier as the feedback resistor thereof. The bandwidth adjuster has a PIN diode, the bias current of which is controlled by the control signal to vary the dynamic resistance thereof Accordingly, the trans-impedance including the PIN diode varies depending on the transmission rate to optimize the bandwidth of the trans-impedance amplifier. The control signal for the PIN diode is provided from the external CPU. The CPU may decide the transmission rate by extracting the clock involved within the data signal, and output the control signal to the PIN diode depending on thus detected transmission rate.
Typical receiving optical subassembly, which is often called as a ROSA, installs a preamplifier within the common package in addition to the light-receiving device when the transmission rate exceeds 1 Gb/s (gigabit per second). The output buffer of the preamplifier often has a CML (current mode logic) configuration, and a limiting amplifier or occasionally a main amplifier is placed in the down side of the preamplifier. For such high speed transmission rate over 1 Gb/s, the input impedance of the limiting amplifier, or that of the main amplifier must be identical with the transmission impedance of the line connected between the preamplifier and the main amplifier to eliminate the failure derived from the impedance miss-matched effect. The impedance of the transmission line is typically 50 ohm, when the line is a differential line, then the impedance thereof is set 100 ohm.
On the other hand, for the low transmission rate around 100 MHz, to shorten the transmission line may eliminate the impedance miss-matching effect. Moreover, when the input impedance of the down side amplifier is kept low, as for the high transmission rate, the total gain of the optical receiver becomes insufficient. A coupling capacitor is often inserted between the preamplifier and the downside amplifier to cut the DC level. This coupling capacitor constitutes a low cut filter together with the input impedance of the downside amplifier. When the input impedance is rigid and optimized for the high transmission rate, the low cut-off frequency required for the low transmission rate can not be obtained.